'Washington Post' reports FBI considers problem a challenge because recruits could return to US to perpetuate acts of terror.

The FBI said that it considers Americans fighting in the Syrian civil war a “growing problem for US intelligence and law enforcement,” The Washington Post reported Friday.

The report said the FBI’s central concern was that Americans who affiliate themselves with al-Qaida- linked rebel forces in Syria could return to the US and perpetrate acts of terror.

The FBI source did not give the Washington Post an estimate of how many Americans are believed to be fighting in Syria. However, a study by the Meir Amit Intelligence and Terrorism Information Center at the Israeli Intelligence and Heritage Commemoration Center estimated the number of foreign combatants fighting in Syria at between 6,000 and 7,000.

Researchers, who spent several months putting together the study, warned that in Syria, foreign jihadis gain military experience and undergo a process of radicalization.

The two central jihadi organizations, the Nusra Front, headed by al-Qaida’s central leader Ayman al-Zawahiri, and its competitor the Islamic State in Iraq and Greater Syria – a branch of al-Qaida in Iraq – have a combined membership of 9,000, of which an estimated 6,000 are foreign volunteers, according to the study.